In optical fiber transmission system, dispersion effect, as a result of different frequency components or various transmission rates for various modes of frequency components, may severely affect signals traveling in the optical fibers and thus distort signal waves and cause inter-symbol interference. The damage incurred by the dispersion on the system performance cannot be ignored. Generally, an optical fiber transmission system over 10 Gbit/s requires a dispersion compensation to guarantee the transmission function of the system. Currently, a widely-commercialized dispersion compensation technique utilizes dispersion compensation optical fibers which are contrary to the dispersion characteristics of the transmission optical fiber to realize dispersion compensation. In recent years, due to the long distance dispersion compensation capability and self-adapted compensation capability, an electrical dispersion compensation technique at a transmitting end overcomes the defect of using dispersion compensation optical fibers and thus draws wide attention of the industry. However, one problem of the dispersion pre-compensation is that it may introduce a considerable peak power to average power ratio at the transmitting end, which may cause the signal to suffer from a severer non-linear effect during transmission. The non-linear effect herein mainly refers to self-phase modulation effect, cross-phase modulation effect, four-wave mixing effect and the like in optical fibers. The influence of the non-linear effect is associated with signal transmission distance, launching power and signal waveform. The non-linear effect on the optical fiber transmission system over 10 Gbit/s cannot be ignored. Currently, an effective way to suppress non-linear effect is to employ a new modulation mode, for example, Return-to-Zero (RZ), Carrier-Suppressed Return-to-Zero (CSRZ), Chirped Return-to-Zero (CRZ).
A block diagram of a transmitter capable of electrical dispersion pre-compensation in the prior art is illustrated in FIG. 1. The transmitter encodes the data signal electrically first, and generates Carrier-Suppressed Return-to-Zero electrical signals with various pulse widths. Then, a pre-compensation module is used to perform dispersion pre-compensation on the Carrier-Suppressed Return-to-Zero electrical signals to generate digital pre-warped electrical signals. The digital pre-warped electrical signals are converted to dispersion pre-compensation CSRZ electrical signals via a digit-analog converting module. Finally, the dispersion pre-compensation CSRZ electrical signals are processed by an electro-optic modulator and output as pre-warped optical signals. The pre-warped optical signals are used to compensate the dispersion effect caused by the optical fiber lines coupled to the electro-optic modulator. However, the CSRZ electrical signal introduced by the coding module may broaden the bandwidth of the data signal. For a system over 10 Gbit/s, the bandwidth of the current digit-analog conversion module cannot fulfill the bandwidth requirement.
FIG. 2 illustrates a conventional chart of the spectrum of a dispersion pre-compensation CSRZ electrical signal, where the x-axis and y-axis indicates frequency and power of the CSRZ electrical signal
            s      ⁡              (        t        )              ⁢    sin    ⁢                  ⁢                  ω        b            2        ⁢    t    ,respectively. The CSRZ electrical signal has a duty cycle of 67%, with a main lobe width of 15 GHz, which is 50% broader than the main lobe width of the conventional Non Return-to-Zero symbols. As a result, the digit-analog converter is required to have a bandwidth of at least 30 GHz. Therefore, the bandwidth of the electrical device such as digit-analog converter required by the electrical dispersion compensation technique has to be increased accordingly. However, the existing digit-analog converter cannot meet the requirement. Therefore, introducing the Return-to-Zero symbols to electrical dispersion pre-compensation technique for suppressing non-linear effect cannot be realized in the prior art.